It is reported that Mufaddal b. Muhalhal – Allāh have mercy on him – said:
If the heretic started mentioning his bid’ah right at the beginning of the sitting, you would be on your guard and flee from him, but what he does is begin by mentioning ḥadīth from the Sunnah. Then, he slips his bid’ah in on you. It might then stick to your heart; but when will it leave your heart?

It is reported that Khālid b. Ma’dān – Allāh have mercy on him – said:

There is not a person except he has four eyes: two in his head with which he sees the matters of this worldly life, and two in his heart with which he looks to the matters of the hereafter. So if Allāh wants good for his servant, He opens his heart’s eyes, and so he perceives what he has been promised in the unseen world. Thus he is saved from the unseen [punishment] through the unseen [reward for the obedient].

The likeness [of the munāfiq] is as the likeness of one who kindled a fire; then, when it lighted all around him, Allāh took away their light and left them in darkness. So they could not see. They are deaf, dumb and blind, so they return not [to the Right Path]. (Quran 2:17)

Qatādah – Allāh’s mercy be upon him – said:

This is the likeness Allāh has given of the Hypocrite (munāfiq); he says lā ilāha illallāh and with it marries into the Muslims, inherits from the Muslims, fights alongside the Muslims, and protects his blood and wealth. But when death comes, [the testimony of faith] has no basis in his heart, and no reality in his actions, so the munāfiq is stripped of it at death and left in darkness and blindness wondering therein, just as he was blind about the right of Allāh and obedience to Him in his worldly life and deaf to the truth.